ABSTRACT of the Doctoral Thesis
The present thesis is concerned with the computational simulation of laminates made from fiber reinforced polymers, as they are nowadays increasingly used in structural components. These laminates are stacks of layers of a matrix material reinforced by uni-directional fibers. The objective of this work is to improve predictions of the thermomechanical behavior of such laminates by developing new tools for numerical simulation which can also be employed in structural analysis. To this end, material laws are formulated on the ply level which are focused on reflecting the typical failure mechanisms observed in fiber reinforced plies.
After a general introduction to modelling approaches for laminates and a summary of experimentally observed failure mechanisms reported in the literature, the main portion of the thesis is concerned with two fields of laminate modelling. In chapter 2 the prediction of laminate failure is treated within the framework of the 'first ply failure' concept, while in chapter 3 the simulation of progressive damage, which leads to a gradual change of material properties, is considered.
In the beginning of chapter 2 the state of the art in first ply failure modeling is reviewed. The first ply failure approach is based on the assumption of proportional increase of all stress components with load. To evaluate stress states with constant stress contributions, the superposition method' for combined stress states is adopted and implemented as a `stand alone' tool as well as a post-processing tool combined with a finite element program. As one of the currently most promising failure criteria the Puck criterion, which is based on physical failure mechanisms and Mohr's fracture hypothesis for brittle materials, is briefly introduced. The application of the developed Fortran program in structural analysis is demonstrated by some example problems. As a typical example for combined load cases, the influence of production related stresses superimposed on mechanical service loads is studied.
The simulation of progressive damage in chapter 3 is based on continuum damage mechanics. Several existing damage models for fiber reinforced laminates are discussed and compared in an extensive literature review. Based on the failure mechanisms postulated by Puck a new damage model is developed. The objective is to derive a thermodynamically consistent relation that is able to describe the change of the complete elasticity tensor as a function of damage, capturing the non-isotropic nature of damage in fiber reinforced composites. In view of its practical application the model is designed such that only a relatively small number of parameters is required which can be identified from standard test data.
On the one hand, the damage model is combined with classical lamination theory in order to study the damage behavior of laminates. On the other hand, it is implemented as a constitutive law into a finite element program. This way analyses of complex structures can be performed under consideration of damage. The identification of model parameters is shown for two material systems. For demonstration purposes the presented damage model is applied to some examples of laminates and fiber reinforced structures and the results are compared to experimental data from the literature. Based on the correlation between simulations and experiments the validity of the fundamental assumptions of the damage model are discussed.